The present invention relates to a process, and an apparatus for improving sheet properties in the forming section of a paper making machine, which section includes a forming fabric moving in the machine direction and a head box having a head box slice which delivers a stock jet onto the moving forming fabric. It is particularly concerned with a process and an apparatus in which the scale and intensity of the activity within the stock is adjusted by altering the position of some of the fabric support elements beneath the forming fabric adjacent the head box slice, so that they either contact, or do not contact, the machine side surface of the forming fabric and observing the effect, if any, that the adjustment of the position of the selected support element or elements has on the quality of the paper product being made on the paper making machine. These fabric support elements are located on at least one drainage box. This invention is thus of use in both a conventional papermaking machine forming, section having a single open surface forming fabric, in the initial open surface section of a so-called hybrid gap former having two superposed forming fabrics, and in twin fabric papermaking machines equipped with a curved forming shoe.
In the manufacture of paper and board products, a highly aqueous stock consisting of about 98-99.8% water and from 0.2-2% papermaking fibers and other solids is ejected at high speed from a headbox slice onto a moving forming fabric. Adjacent the head box slice, the forming fabric passes in sliding contact over a plurality of static fabric support elements which serve to support the forming fabric, and to define a reference surface over which the forming fabric moves. Depending on the surface profile chosen for the fabric support elements, they may also assist in draining water from and generating turbulence in the stock on the forming fabric. The fabric support surfaces usually include a lead blade located more or less underneath the point at which the stock jet impinges the forming fabric, followed downstream by at least two additional surfaces, each of which may be flat, or profiled to act as foils (e.g. as disclosed by Wrist in U.S. Pat. No. 2,928,465) or as agitators (e.g. as disclosed by Johnson, in U.S. Pat. No. 3,874,998), although stock agitation is not typically initiated at this very early point in a forming section. In current practice, the individual fabric contacting elements adjacent the head box slice are not vertically adjustable after installation (see for example Rulis et al., U.S. Pat. No. 6,274,002). These elements are normally mounted onto the supporting structure using either a dovetail, or a T-bar, and in some cases are solidly mounted. The mountings are also arranged on the supporting structure so that all of the elements are in permanent non-adjustable contact with the forming fabric. Stock activity can only be adjusted by changing, or physically removing, the fabric contact elements by sliding them out of their dovetail or T-bar mount. It is both very difficult and time consuming to carry out such a change in the configuration of the forming section fabric supporting elements while the paper making machine is running, bearing in mind that the forming fabric can be moving at up to 100 kph.
Good sheet properties result from the injection of kinetic energy into the stock, which causes the papermaking fibers to become agitated and thus relatively more uniformly dispersed. This is particularly important in the early part of the forming section adjacent the head box slice, where the paper making fibers are still relatively mobile.
Paper makers are currently seeking means to accommodate changes in the basis weight of the paper product being manufactured by the same machine. For example, a papermaking machine producing liner or board grades at a relatively low basis weight may have to shift to a far higher basis weight product which requires a much slower machine speed. This change in speed reduces the beneficial stock turbulence and adversely affects product quality. It is thus apparent that a given papermaking machine, without some means of adjustment, is best at making one grade of paper product and cannot readily be altered to make a significantly different one. It is not uncommon today for a papermaking machine to have to accommodate a doubling or tripling of the basis weight of the product being manufactured. Such changes are frequently difficult to manage without compromising sheet quality, and therefore alternative means of achieving a required level of activity in the stock are necessary to accommodate changes in paper grade with minimal disruption to production.
This invention seeks to provide a method and an apparatus by means of which the paper maker can alter the number of fabric support elements in contact with the forming fabric in the area immediately after the stock impinges onto the forming fabric adjacent to the head box slice so as either to enhance, to maintain or to diminish stock activity and thereby optimize agitation in accordance with papermaking conditions to provide a product of acceptable quality. It also offers the possibility that a contact element with a given profile can be vertically adjusted to be out of contact with the forming fabric and a support element with a different profile vertically adjusted to be in contact with the forming fabric. It further offers the opportunity to change the support element so that one element profile can be changed for another while the element is not in contact with the forming fabric. Since the effect, if any, of adjusting a particular support element can only be determined after the paper making conditions have stabilized and product made under the new conditions retrieved from the paper making machine and examined, there inevitably is a time lag between a support element being adjusted and the consequences of that adjustment being known. This time lag generally will be at least several minutes, but can be up to an hour.
Smith et al. in U.S. Pat. No. 5,080,760 disclose a sealed pressurized forming board in which the rate of drainage of fluid from the stock is slowed by applying a small positive pressure to the machine side of the forming fabric, which also allows stock activity to be initiated without increasing drainage. The level of stock activity can only be controlled by physically removing the fabric supporting elements and replacing them with others having differing support surface profiles.
Johnson in U.S. Pat. No. 4,140,573 discloses a sealed cover for a low vacuum suction box in which all of the fabric contacting elements shown have an essentially flat surface in contact with the fabric and some of the fabric contacting elements are fixed in a position that is a small amount below the others. This unit requires an applied vacuum which is sufficient to cause the forming fabric to contact all of the element surfaces so that the forming fabric follows an undulating path, thereby inducing agitation in the stock. Johnson discloses that the distance below the normal plane of the fabric of some of the elements can be adjusted by means of pins and set screws, or by movement of an element having a sloping T-recess relative to sloping a T-bar. This box is not generally used adjacent the headbox slice, since vacuum assisted drainage is usually not desirable at that early point in the forming section.
Miller in U.S. Pat. No. 5,421,961 discloses a forming board whose cross-machine direction orientation relative to the headbox is adjustable and controlled so as to maintain it in parallel relation to the slice. There is no effort or intention to control stock activity.
Thorp in U.S. Pat. No. 4,443,298 discloses an adjustable blade support member whereby, in one embodiment, the blade-to-blade spacing in the machine direction is adjusted by means of a screw. However, the elements are only moved laterally, not vertically.
Ibrahim in U.S. Pat. No. 4,684,441 and U.S. Pat. No. 4,718,983 discloses a forming board in which the width of the lead-in blade is adjustable without stopping the papermaking machine. A thin strip located on the paper side surface of the blade can be moved so as to intercept the point of impingement of the stock jet.
Morley in GB 2,190,932 discloses a forming board in which the depth of a slot or groove can be changed by physically removing or inserting spacers of different thicknesses, without stopping the papermaking machine so as to induce microturbulence.
Goddard in U.S. Pat. No. 4,321,108 describes a forming section in which the fabric supporting structures are carried by supporting rails extending along the length of the forming section and spaced across its width. The fabric supporting structures are attached to the rails by a clamping means such as a C-clamp or a J-clamp. Since adjustment of the spacing of the fabric supporting structures in the machine direction requires access to the clamping means, the machine has to be stopped to make any adjustments.
Tokuno, in U.S. Pat. No. 5,456,803 describes a twin fabric forming section in which at least one side of the forming fabric is contacted by table rolls so arranged as to deflect the fabrics as they pass over them. By causing the forming fabrics to move in an undulating path pressure pulses are generated in the stock which apply shear forces to the paper web to dewater it. A disadvantage with this forming section appears to be the use of multiple table rolls; these are known to cause significant problems in modern fast papermaking machines.
Rulis, in U.S. Pat. No. 6,274,002 describes a paper making machine in which several support elements in the paper making machine forming section can be moved by means of a mechanical pin-and-slot mechanism either vertically, or to alter the orientation of the contact surface of the element relative to the machine side surface of the forming fabric. The positions of one, or more, support elements are changed in response to variations in preselected paper making condition parameters which are provided by suitably located sensors. Rulis is typical of many proposed systems in that a mechanical drive system located beneath the forming fabric is used to alter the position of the selected support element. Experience shows that as these drive devices are exposed to the white water draining through the forming fabric their long term reliability is very questionable, due to a coating of paper making solids which forms rapidly on them.
Ekberg, in U.S. Pat. No. 3,864,207 describes a forming section in which at least one of the fabric supporting devices is flexibly supported, for example by means of an elastomeric block. By flexing the support, a fabric contacting surface is caused to move rapidly and continuously through a more or less elliptical path. In the upper part of the path the surface is in contact with the forming fabric and urges it in the machine direction. In the lower part of the path the surface is not in contact with the forming fabric and moves in the opposite direction. The device moves through the support element through this elliptical path quite rapidly, at a rate of from about 1,000 to about 4,000 strokes/minute. It is thus apparent that the fabric supporting surface is only in contact with the forming fabric for part of the more or less elliptical path. The device is alleged to reduce the power requirements to move the forming fabric in the machine direction, and to promote agitation within the stock on the forming fabric.
A number of inventors have disclosed vertically adjustable support element mounting devices. Typical structures are shown by: Bartelmuss et al in U.S. Pat. No. 5,660,689; Bubik et al. in U.S. Pat. No. 5,262,010 and U.S. Pat. No. 5,061,347; and Jaakkola U.S. Pat. No. 5,387,320.
This invention seeks to provide both a process, and an apparatus, whereby agitation in the stock on the forming fabric adjacent to the head box slice can be readily altered by the paper maker, thus providing some level of quality control over the formation in the product being made, without having to stop the paper making machine for adjustments to be carried out. It is thus possible to initiate agitation within the stock, and to adjust both its scale and intensity, adjacent to the headbox slice. This is achieved by mounting at least some of the fabric support elements on vertically adjustable support means so that their height can be altered so as to contact, or not to contact, the machine side of the forming fabric. Further, the apparatus envisaged by this invention allows the element vertical position to be adjusted so that the element is either moved into contact, or moved out of contact, with the forming fabric without stopping the paper making machine.
Thus in a first broad embodiment this invention seeks to provide a process for improving sheet properties in a paper making machine including a forming section, at least one forming fabric, having a machine side and a paper side, moving in the machine direction and a head box having a head box slice which delivers a stock jet onto the paper side of the at least one moving forming fabric, comprising the steps of:
(i) discharging the stock jet onto the paper side of the at least one moving forming fabric;
(ii) causing the machine side of the at least one forming fabric carrying the stock on its paper side to move over in sequence in the machine direction:
a first fixed support element adjacent the head box slice and in contact with the forming fabric;
at least one drainage box provided with a foraminous support surface, which support surface includes at least one vertically adjustable support element, and provided with support means for the first fixed element; and
a last fixed support element in contact with the forming fabric, which together with the first fixed support element and the machine side of the at least one forming fabric defines a reference surface; and
(iii) adjusting both the scale and intensity of agitation in the stock at at least one locus by adjusting the vertical position relative to the reference surface of at least one vertically adjustable element whereby the at least one element is adjusted vertically into a position chosen from the group consisting of in contact with the machine side of the forming fabric, and out of contact with the machine side of the forming fabric, and
(iv) continuing to operate the forming section according to steps (i) and (ii).
Preferably, the process further includes:
(v) after a lapse of time sufficient for stable conditions to be re-established in the forming section following step (iii);
(vi) assessing the sheet properties; and
(vii) if required repeating step (iii) to improve sheet properties further.
Preferably in step (iii) a first element is adjusted to be out of contact with the forming fabric and a second element is adjusted to be in contact with the forming fabric.
Alternatively, in step (iii) a first element is adjusted to be out of contact with the forming fabric and a second element is adjusted to be in contact with the forming fabric, and the two elements have the same fabric contacting surface profile.
Alternatively, in step (iii) a first element is adjusted to be out of contact with the forming fabric and a second element is adjusted to be in contact with the forming fabric, and the two elements do not have the same fabric contacting surface profile.
In a second broad embodiment this invention seeks to provide an apparatus for improving sheet properties in a paper making machine including a forming section, at least one forming fabric, having a machine side and a paper side, moving in the machine direction and a head box having a head box slice which delivers a stock jet onto the paper side of the at least one moving forming fabric, comprising in sequence in the machine direction:
a first fixed support element adjacent the head box slice and in contact with the machine side of the at least one forming fabric;
at least one drainage box provided with a foraminous support surface, which support surface includes at least one vertically adjustable support element, and provided with support means for the first fixed element;
a last fixed support element in contact with the machine side of the at least one forming fabric, which together with the first fixed support element and the machine side of the at least one forming fabric defines a reference surface; and
an independently controlled vertical adjustment means for the at least one vertically adjustable support element which permits adjustment of the vertically adjustable element while the paper making machine is operating.
Preferably, the last fixed element is supported by an adjacent drainage box in the forming section. Alternatively, the last fixed element is supported by the at least one drainage box.
Preferably, the apparatus includes one drainage box. Alternatively, the apparatus includes two drainage boxes, each of which supports part of the foraminous surface.
Preferably, the foraminous surface includes a plurality of support elements, at least one of which is vertically adjustable, and the remainder of which have their fabric support surfaces located along the reference surface. More preferably, the foraminous surface includes a plurality of support elements, at least some of which are vertically adjustable while the paper making machine is operating, and the remainder of which have their fabric support surfaces located along the reference surface. Alternatively, the foraminous surface includes a plurality of support elements, all of which are vertically adjustable independently of the others while the paper making machine is operating. Conveniently, the foraminous surface includes either a plurality of support elements in which alternate elements are vertically adjustable, or a plurality of support elements in which alternate elements are vertically adjustable, and in which the support elements adjacent the first and last fixed elements are adjustable.
Preferably, in the forming section the profile of the supporting surfaces of the elements are all the same.
Alternatively, in the forming section the profile of the supporting surfaces of the elements are not all the same.
Alternatively, in the forming section the profile of the supporting surfaces of the vertically adjustable elements are all the same.
Alternatively, in the forming section the profile of the supporting surfaces of the vertically adjustable elements are not all the same.
Preferably, at least that portion of the forming section adjacent the head box slice has an open surface. More preferably, the papermaking machine including the forming section is chosen from the group consisting of:
an open surface papermaking machine having a single forming fabric;
a hybrid gap former papermaking machine having two superposed forming fabrics in which at least that portion of the forming section adjacent the head box slice has one forming fabric having an open surface; and
a twin fabric papermaking machine having two forming fabrics each having a paper side and a machine side, in which the jet of stock is delivered into the gap between the paper sides of the two fabrics, and including at least one drainage box in which at least one support element in contact with the machine side of one of the forming fabrics is moveable.
In the context of the direction of adjustment of the adjustable forming fabric support elements used in this invention, the word xe2x80x9cverticalxe2x80x9d is to be taken to mean a direction essentially perpendicular to the machine side of the forming fabric adjacent to that element. In an open surface forming section, either as part of a single forming fabric machine or as the initial part of a hybrid gap former, this generally will be in a truly vertical direction, as the forming fabric open surface is horizontal. In the forming section of a twin fabric papermaking machine, this will often be a direction other than truly vertical, since the forming sections in such machines are often not horizontal.